Prograde and radial

[bradwiggo]

I don't understand the difference between prograde/retrograde and radial/anti-radial. 

I don't know when to use radial and what it does. 

When and why would I want to burn radial, and why radial not prograde or retrograde? 

[MachTurtle]

I'll try to provide a quick answer.  When you're in an orbit and accelerate prograde, you increase the altitude of your orbit 180 degrees (opposite side of the body you're orbiting). If you accelerate retrograde, it decreases the altitude of the orbit there.  Accelerating radially will precess (shift) the orbit forward while accelerating antiradially will move it backward.  I'm in a rush so I apologize if I'm not being clear and trust others will be more thorough.

Good luck!

[Starhawk]

Radial is the direction directly away from from the body you are orbiting.  Anti-radial is directly toward the body you are orbiting.

Burning radial and anti-radial change your orbit in other ways than burning prograde/retrograde.  Essentially, your orbit pivots around the point where your ship is located.  I generally use it in combination with prograde/retrograde to fine tune encounters during interplanetary transfers.

I would recommend putting something in orbit and then playing around with various maneuvers to see how they affect your orbit.

Happy landings!

[pincushionman]

Pro-/retrogradeaffects the point in the orbit 180 degrees from your current position.

(anti-/)radial affects the points 90 degrees behind/ahead of your current position.

it is less efficient than a properly-implemented pro-/retrograde burn. But there are uses. A radial component to a circularization burn can save you from a less-than-ideal suborbital trajectory after you've missed apoapsis. Also, if on an impact trajectory with a body and are still a long ways off, a small radial burn may be more effective to nudge your orbit out of the lithosphere, depending where exactly you want it to be.

[Kyrt Malthorn]

Assuming your navball is in Orbit mode (as opposed to Surface or Target):

 

Prograde - the direction you are going.  Retrograde is opposite, the direction you just came from.  These are both yellow markers.  If you're in a nice equatorial orbit and timewarp a while, you may notice your prograde and retrograde markers scroll around the navball along the center line.  This is because the path of your orbit is roughly a circle, and that direction changes depending on what point of that circle you currently occupy.  Burning prograde or retrograde is usually the most efficient way to modify your orbit.  Ideally, you want to wait until the point in your orbit that burning that direction will have the desired effect.

 

Radial - comes from the same word as Radius (the line from the center of a circle to the edge) which may help you visualize this.  Draw a line from the center of your orbit to your ship.  The Radial direction is following the direction of that line out into space.  Anti-radial is the direction to the center of your orbit.  It's easy to see this if you're in a circular orbit, because the anti-radial marker will always be directly aimed at the planet (centered of the orange half), while anti-radial will always be aimed directly away (centered of the blue half).

Honestly, most times I use Radial burns I end up regretting it.  It's very inefficient compared to a prograde or retrograde burn somewhere else your orbit.  I mostly find myself using it to fine-tune intercepts, such as when I'm coming to the Mun and I've already burned prograde until I saw I would enter the Mun's sphere of influence.  A tiny radial burn may adjust how close to the Mun I pass, so I don't have to mess with my orbit as much when I get there.  I always do this with a maneuver node so I know exactly what it will do for me.

In fact, maneuver nodes are a great way to learn these directions.  The markers on each axis of the maneuver node tool are the same as the markers on your navball.  They also point in the directions they represent, so you can see which way is prograde at any given point along your orbit if you put a maneuver node there. Play around with the maneuver nodes and see if it helps you get a more intuitive feel for what these kinds of burns do.   I know it's hard to grasp why this stuff works the way it does.

 

Good luck!

 

 

Edited March 29, 2016 by Kyrt Malthorn
I can spell. Usually.

[FancyMouse]

6 minutes ago, Starhawk said:

Radial is the direction directly away from from the body you are orbiting.  Anti-radial is directly toward the body you are orbiting.

Actually it's directly away/towards only for circular orbit, or at Pe/Ap of an elliptic orbit. On minor axis of a pretty eccentric orbit, for example, there's an obvious decent angle.

[N_Danger]

I sometime use a radial or anti-radial component to adjust an orbit for rendezvous. To make the orbit at intercept tangent to the target's orbit so that when I match courses I end up almost in the same orbit within a few KMs of the target.

 

[Plusck]

One thing that's vitally important (in my humble opinion) to understand what different burns do, is that whatever direction you burn in you will always return to your current position exactly one orbit later unless you make another burn somewhere.

And any change at one point in the orbit will lead to an equal and opposite change at the other side of your orbit (half an orbit away), and make the biggest real difference* to your orbit a quarter of an orbit away.

(* note: you could say that this is the opposite for a prograde/retrograde burn which we think of because of it's effects at 180°, not 90° later, to which I would respond that no: all characteristics of the orbit are radically different 90° later: angle compared to surface, position in the sky compared to surface, speed and distance. 180° later it is only speed and distance which are different)

So if you make a radial burn, the only thing that changes is the angle of your present trajectory. If you head "inwards" from a circular orbit, your orbit heads downwards at that point. That means that a quarter of an orbit away you will be closer to the body (and therefore faster than if you hadn't made a radial burn), at the opposite side your angle will head away from the body, and three quarters of an orbit later you will be further out and slower. Assuming your orbit is eastwards, then looking from above (from the north) you will have made your orbit "slant" to the left, rotating about your present position which cannot change.

Similarly with prograde: you speed up at your current position, so you will be slower at the opposite side, and the most significant change is a quarter of an orbit later (your trajectory will be angled further away from the body) or three-quarters of an orbit later (you'll be angled in towards the body).

So obviously, prograde at one point in the orbit is very similar in effect to radial-out a quarter of an orbit later, which is similar to retrograde a quarter of an orbit after that, which is similar to radial-in a quarter orbit after that, and so on.

The main difference is efficiency: one prograde burn is the same as doing two slightly smaller (and opposing) radial burns a quarter and three-quarters of an orbit later: so if all you want to do is raise your Ap, it is far more efficient to do one prograde burn. Conversely (although the situation arises less frequently), if you want to change Ap and Pe in opposite directions (such as to circularise to match a satellite's orbit as you cross it) one radial burn wil probably be more efficient than two separate prograde/retrograde burns at Ap and Pe. The only reason why that might change is due to the Oberth effect, which is another subject entirely.

[Waxing_Kibbous]

The radial nodes are good for making that perfect orbital capture node- next time you set up an orbital capture node, after you pull the retrograde node to circularize pull the radial ones to see what they do. One tip that has stuck in my head concerning radial nodes is that they are similar to what a hula hoop does.

[Reactordrone]

Here's a quick visual of the results from the different burn directions. Burning prograde or retrorade increases or decreases the height of the opposite side of your orbit. Radial and anti-radial rotate your orbit around your current location.

Edited March 29, 2016 by Reactordrone